Recently, to improve users' convenience and satisfaction, communication providers have given high priority to the reduction of blind areas such as shadow of buildings in downtown, the insides of restaurants, cell edges, and high-rise buildings where radio waves of mobile stations such as mobile phones can not sufficiently reach.
In an area that does not have an enough space for the facility of a base station or a mountain area where the number of users is small, repeaters (relay devices) that perform a relaying operation that amplifies signals transmitted from base stations and mobile stations and re-transmits the amplified signals are often installed instead of new base stations.
However, since relay devices also amplify not only signals received from base stations and mobile stations, but also interference waves and noises received together with those signals and relay the resultant signals, the relay devices may lead to an adverse affect upon the system such that they deteriorate signals received by base stations and neighbor mobile stations.
Thus, it is desirable that while mobile stations in the neighborhood of a relay device are not communicating with a base station, the relay device needs to restrict the relaying operation whenever possible.
To do that, recent relay devices are controlled such that they perform the relaying operation only if the total reception power of all uplink signals exceeds a reference power level that has been set for them.
Radio communication systems that have been implemented or evaluated (hereinafter it is assumed that they includes mobile phone systems or the like) use a transmission scheme in which a wide system frequency band is ensured and segmented for individual mobile stations when necessary.
For example, in LTE (Long Term Evolution) that is being standardized in 3GPP (Third Generation Partnership Project), a frequency band that is as wide as a maximum of 20 MHz is segmented into 100 resource blocks and also chronologically segmented into short periods. The resource blocks are assigned to control channels and data channels.
Thus, in a high traffic area, the entire system frequency band is always used. In contrast, in a low traffic area, part of the system frequency band is used for a short period.
Consequently, under the LTE's system environment, when the relay device controls the relaying operation based on the total reception power of all uplink signals, namely the total reception power of the entire system frequency band, if the number of mobile stations in the neighborhood of the relay device is very small, even if a mobile station transmits an uplink signal, since the total reception power of the uplink signal that the relay device receives does not exceed the reference power level, the relay device may thus not correctly perform the relaying operation.
To solve such a problem, it can be contemplated that a relay device extracts an uplink signal in a frequency band that a mobile station uses for an uplink channel and performs the relaying operation based on only the reception power of the extracted uplink signal according to a narrow band signal extracting technique presented in Patent Literature 1.